1. Field of the Invention
The present invention relates to optical glass and, more particularly, optical glass having low dispersion and anomalous dispersion, glass material for press molding formed of the glass, an optical element blank, an optical element and manufacturing methods thereof.
2. Description of Related Art
It has been known that, since fluorophosphate glass has low dispersion and anomalous dispersion, fluorophosphate glass can be suitably used as material for lenses used for correction of high-order chromatic aberration. Japanese Examined Patent Application Publication No. HEI 4-43854B (hereafter, referred to as JP HEI 4-43854B) discloses fluorophosphate glass having low dispersion as well as a high refractive index.
However, such low dispersion glass has drawbacks. In particular, since the fluorophosphate glass exhibits a remarkably high degree of volatility in a molten state, it is difficult to obtain glass having a high degree of optical homogeneity from the fluorophosphate glass. Also, although the glass disclosed in JP HEI 4-43854B has excellent anomalous dispersion, the glass still has room for improvement of the thermal stability. Regarding glass whose thermal stability is low, it is necessary to increase the temperature of flowing molten glass to prevent the glass from losing clarity when the molten glass is molded. In this case, the volatilization is promoted. Therefore, it becomes more and more difficult to obtain the glass having the high degree of thermal stability.